fix(controllers): allow stateful feedforward in Feedforward2Dof (#277) (#281)

Remove const from Feedforward::Evaluate(), add pure-virtual Reset() to
the interface (consistent with FeedbackLaw::Reset()), and delegate
feedforward.Reset() from Feedforward2Dof::Reset() so dynamic pre-filters
that carry internal state are correctly cleared on reset.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gabrielfrasantos

doc/controllers/Feedforward2Dof.md

@@ -55,6 +55,7 @@ If the system were already tracking perfectly ($y = r = 0.5$), then $e = 0$, $u_
 - **Feedforward does not aid disturbance rejection.** If $u_{ff}$ is tuned for tracking and a disturbance enters the plant, only $u_{fb}$ responds.
 - **Inverse-plant feedforward for non-minimum-phase systems** is non-causal; use a truncated or approximate inverse.
 - **Sign convention.** The error passed to feedback is $r - y$, not $y - r$. Swapping signs destabilises positive-gain plants.
+- **Stateful feedforward must implement `Reset()`.** Dynamic pre-filters (e.g. reference model filters) carry internal state that must be cleared on `Reset()`. Both the feedforward and feedback components are reset when `Feedforward2Dof::Reset()` is called; any feedforward implementation that holds state must override `Reset()` to clear it.
 
 ## Variants & Generalizations
 

numerical/controllers/implementations/Feedforward2Dof.hpp

@@ -17,7 +17,8 @@ namespace controllers
         static_assert(std::is_floating_point_v<T>, "Feedforward supports floating-point types");
 
         virtual ~Feedforward() = default;
-        virtual T Evaluate(T reference) const = 0;
+        virtual T Evaluate(T reference) = 0;
+        virtual void Reset() = 0;
     };
 
     template<typename T>
@@ -68,6 +69,7 @@ namespace controllers
     template<typename T>
     void Feedforward2Dof<T>::Reset()
     {
+        feedforward.Reset();
         feedback.Reset();
     }
 

numerical/controllers/implementations/test/TestFeedforward2Dof.cpp

@@ -9,7 +9,8 @@ namespace
         : public controllers::Feedforward<float>
     {
     public:
-        MOCK_METHOD(float, Evaluate, (float), (const, override));
+        MOCK_METHOD(float, Evaluate, (float), (override));
+        MOCK_METHOD(void, Reset, (), (override));
     };
 
     class MockFeedbackLaw
@@ -109,8 +110,9 @@ TEST_F(TestFeedforward2Dof, zero_feedback_reduces_to_feedforward)
     EXPECT_NEAR(result, 0.4f, math::Tolerance<float>());
 }
 
-TEST_F(TestFeedforward2Dof, reset_delegates_to_feedback)
+TEST_F(TestFeedforward2Dof, reset_delegates_to_feedforward_and_feedback)
 {
+    EXPECT_CALL(ff, Reset()).Times(1);
     EXPECT_CALL(fb, Reset()).Times(1);
 
     controller.Reset();